Abstract

An elliptic jet with mixing tabs was investigated using the digital particle-imagevelocimetry and two-component hotwire anemometry. Two tabs of cylindrical configuration were placed on the minor-axis sides of a 2:1 elliptic nozzle. Because of their sideways pressure-relieving effect, a cylindrical tab relieves the constraint on the flow relative to a thin tab with flat surfaces which can have important implications on tab drag reduction but with similar mixing characteristics. Measurements were made for a jet exit velocity of 20 m s−1 and for Reynolds number based on the nozzle equivalent diameter (De) of 5.08 × 104. Furthermore, the effect of tab height (h/d = 1.0 and 1.67) is also investigated. The study shows the evolution of a mushroom structure behind each tab that distorts the jet flow into a two-finger structure. The tab wake mainly comprises of base vortices and tip vortices that are enveloped from the sides by spanwise vortical structures.While the strong upwash from the base vortices causes an inward penetration of the wake inhibiting the jet growth along the minor-axis plane, the net flow induced by the vortices shed from the tab free-end and the base region promotes jet growth along the major-axis plane, thereby preventing axis-switching. The combined effect of the above induces a rapid inward and lateral growth of spanwise generated vortices which begin to show at some downstream distance along the jet centreline that marks the end of the jet potential-core and the beginning location of jet-core bifurcation. An increase in the tab height shifts this location further upstream and promotes still higher jet growth along the major-axis plane. The intense initial mixing process initiated by the cylindrical tabs causes a rapid increase in the wake width and a subsequent increase in the overall turbulent intensity.